This invention relates to a method of transmitting an image and to apparatus for carrying out the method.
It has been proposed to transmit coded alphanumeric and graphical data using the line periods at the start of television frames which do not carry video information so that access to a certain number of pages of information is available to television viewers having suitably modified television receivers. This information system is known by the name Teletext. A similar system has been proposed in which information from a much larger data base is accessible via the public switched telephone network. This system is known by the term Viewdata and a service based on it has been started by the Post Office.
Although the Teletext and Viewdata systems have a graphics capability enabling them to produce simple drawings in a display page, it would clearly be a useful improvement to provide facilities for the display of pictures with the alphanumeric information. However, a difficulty arises in the transmission of pictures in a digital format similar to that used for Teletext and Viewdata information in that to provide a picture of the same resolution as a normal television picture would require between 300 and 500 times as much transmission time as is needed for a page of alphanumeric information, and this is clearly unacceptable. One way in which to reduce both the time necessary to transmit the information and the storage required to display it is to decrease the area of the picture and as a result the number of picture elements in it. For example, a picture 1/4 of the height of the screen and 1/4 of its width would occupy a sixteenth of the area and would therefore need only 1/16th of the time for its transmission compared with that for a full screen picture. By the use of sophisticated data compression techniques it is possible to reduce the transmission time further so that such a picture could be transmitted in the time required for 8 text pages and the storage needed reduced similarly.
It would be possible to start with a relatively small picture and produce the video signals directly from it, but this would require elaborate optical equipment to ensure that the timing of the resulting video signals is such as to produce the required small image in a page of Teletext or viewdata information. It is clearly preferable to use a full screen image and select the picture elements to be transmitted by a suitable sub-sampling process, but such a process can give rise to aliasing problems as a result of the original image containing frequency components of too high a frequency relative to the sub-sampling frequency. This could be overcome by band limiting the video signal derived from the original picture, but such band limiting has the effect of degrading the image produced subsequently from the samples and it moreover has the disadvantage that it is not effective in a vertical direction where the aliasing problems may still occur.
For achieving the compression of data representing transmitted images it is known to subject the image to a two-dimensional transform such as, for example, a Fourier transform applied in two dimensions, and to transmit data representing the transformed image. At the receiver the received data is subjected to the inverse transform which restores the original image. The quantity of data that must be transmitted to produce an image of given definition is less using transform coding than would be the case if direct transmission of the image had been used. Details of such transforms and their use in transmitting images may be found in, for example, "Digital Image Processing" by W. K. Pratt, published by Wiley Interscience, 1978, and in a paper entitled "Hadamard Transform Image Coding" by W. K. Pratt, J. Kane and H. C. Andrews, published in the Proceedings of the IEEE, Volume 57, No. 1, January 1969, pages 58 to 68.